A single living cell is a dynamic system constantly sensing and reacting to external stimuli, and can already be considered as a biological network itself. As the hierarchy upgrades, many cells can form a more complex biological network and demonstrate communication and collective behavior. To unravel the biological network even at the single cell level is still challenging because of its complexity and is a critical subject in fields such as system biology. One of the most important tricks in all experimental science is to effectively vary only one thing at a time. As such, the spatial and temporal precision of the delivery of controlled changes is critical.
Recently, we have witnessed a paradigm shift from extracellular control of environmental stimuli to intracellular control of the actual internal connections themselves, which can potentially provide new insights of the living cellular machinery. As an obvious example, an external stimulus will most likely trigger a cascade of cell signaling via various pathways before its effect is actually received by the intended intracellular party. The response of interest may be completely masked or misinterpreted due to signal loss, attenuation or distortion within the long string of signaling cascade. Therefore, intracellular techniques have the potential to deliver the controlled effectors with much improved spatial, temporal and even molecular precision.
Recent advances in nanoplasmonic technology have enabled new tools for light-gated drug delivery, photothermal therapy, DNA release, inducing protein aggregates, and nanometer scale direct interfacing with intracellular processes using oligonucleotides. A distinct advantage of gold nanoparticle-based approaches compared to lysosome vesicle or other metals is the much better control in coating, or functionalizing, them with thiolated ligands directly or through linker molecules, and its chemical inertness. Colloidal gold nanosphere has been first used as a photothermal agent for therapies and light-gated release of surface coated molecules. With plasmon resonance near 540 nm, in vivo applications were limited however by the strong scattering and absorption of skin, tissue, and hemoglobin at this wavelength. As a result, various colloidal gold nanoparticles of other geometry have been developed, e.g., nanoshell, nanorod, and nanocage, with two primary goals: shifting the resonance into the near-infrared transmission window and increasing the nanoparticle's cargo capacity.
Plasmonic nanoparticles are generally characterized by scanning electron microcopy or dynamic light scattering for size distribution, absorption spectroscopy for both size and plasmonic resonance, and surface-sensitive techniques such as surface-enhanced Raman spectroscopy (SERS) using surface adsorbate or thiolated hydrocarbon as markers. Among these, SERS provides label-free adsorbate identification with the highest nanoparticle-molecule distance sensitivity because only the molecules within a few nanometers of the gold surface can be enhanced. In addition, SERS is arguably the most robust and sensitive technique for real biological applications because it is a background-free measurement assuming the photoluminescence from other interferents is negligible.
Over the past decade, many types of colloidal nanoparticles of various shapes have been developed as shown and described below. All the existing nanoparticles share the same feature, that is, they are solid-core, with nanocage as the only exception, which features an empty void inside a “porous” box. Therefore, only a small fraction, i.e., the molecules absorbed on nanocage walls, could be plasmonically enhanced, rendering single nanocage undetectable by SERS.